A. Field of the Invention
This invention relates to torsional oscillation monitoring and more particularly to circuitry for producing alarms when torsional oscillations of a rotating shaft violate predetermined operating criteria.
B. Description of the Prior Art
One form of a vibration monitoring circuit is disclosed by Foster et al in U.S. Pat. No. 3,455,149. Foster et al disclose circuitry for monitoring displacement of a vibrating structure. A first integrating circuit is provided with different time constants depending on whether a peak value or average value of vibration is being measured. A second integrating circuit with an adjustable rise time is employed to reject transient signals. The output signal from the second integrating circuit is compared against a high limit reference voltage and a low limit reference voltage to actuate utilization devices if the output signal exceeds the high limit reference voltage or is less than the low limit reference voltage.
The above-described Foster et al circuit and other prior art circuits have encountered the problem of determining when predetermined acceptable limits of vibration have been exceeded both in terms of magnitude and duration. Foster et al. and other prior art circuitry employ continuous integration of vibration representative signals in rejecting signals of short duration. The resulting integrated signal is then compared against reference values to determine if the magnitude of the vibration is beyond acceptable limits. Thus, a vibration duration analysis and a vibration magnitude analysis have been made interdependent by the prior art circuitry.
The present invention solves the aforementioned and other problems encountered by the prior art by providing a torsional oscillation alarm system which analyzes the oscillation representative signal itself rather than an integrated value thereof to determine if the oscillation is within acceptable limits. A circuit is provided to generate a low level alarm signal when torsional oscillation exceeds a first predetermined magnitude limit and persists for a predetermined length of time. In addition, a separate rate of decay circuit is employed to determine if the torsional oscillation subsides at a predetermined acceptable rate, with this circuit being activated only after a second predetermined magnitude limit has been exceeded. The present invention utilizes both the value of the oscillation signal which resulted in activation of the decay circuit and the instantaneous value of the oscillation signal to make a determination as to whether the torsional oscillation has subsided at the predetermined acceptable rate.
One of the objects of this invention is to provide a torsional oscillation alarm system to generate an alarm signal if the oscillation representative signal exceeds a predetermined magnitude and fails to subside at a predetermined acceptable rate.
Another object is to provide a torsional oscillation alarm system to generate an alarm signal if an oscillation representative signal exceeds a predetermined limit for a predetermined length of time.